It may not quite be the Tardis, but scientists have built what could loosely be described as a time machine.

In an experiment that would have challenged Doctor Who, researchers defied the second law of thermodynamics, which governs the direction of "time's arrow" from past to future.

Doctor Who's Tardis

Working in the weird realm of quantum mechanics, they achieved the equivalent of causing a broken rack of pool balls to re-order itself.

READ MORE: Power stations and uni hold science dayIt was as if the balls scattered randomly around a pool table went into reverse and packed themselves back into their original pyramid formation.

To an outside observer, it looks as if time is running backwards.

Lead researcher Dr Gordey Lesovik, who heads the Laboratory of the Physics of Quantum Information at the Moscow Institute of Physics & Technology (MIPT), said: "We have artificially created a state that evolves in a direction opposite to that of the thermodynamic arrow of time."

The "time machine" described in the journal Scientific Reports is based around a quantum computer that carries out calculations using basic elements known as superconducting "qubits".

A qubit is a unit of information described by a "one", a "zero", or a mixed "superposition" of both states.

The same kind of effects happen in nature with sub-atomic particles such as electrons occupying more than one state at a time until they are observed or disturbed.

An electron's physical position is defined by uncertainty, meaning that instead of being a concrete "point" it is a fuzzy state of probabilities smeared across a region of space.